This invention generally relates to a direct current to direct current power converter and more particularly to such a converter having improved start and overload characteristics.
Generally, D.C. to D.C power converters have produced selected voltage and power levels by interrupting the incoming current by means of a pair of push-pull switching transistors. In such systems, a commercial power source is rectified and filtered. The filtered D.C. is fed to the switching transitors and raised to the desired level by a suitable transformer. To obtain adequate ripple reduction the rectified output voltage from the transformer generally includes a smoothing choke. Therefore, the cost and complexity of such systems is relatively high if considerable power levels are to be obtained. One such switching converter is described in U.S. Pat. No. 3,843,919 entitled "DC-TO-DC Converter" issued Oct. 22, 1974 to Yamamura et al.
For the purpose of reducing system weight, energy storage converters incorporating a single switching transistor and an air gap multiwinding choke have been suggested. As a result of the high impedance coupling between the switching and output circuits, capacitive filtering may be used, thus eliminating bulky smoothing inductors for output filtering. However, such converters are not generally self-starting and therefore use a separately powered self-starting circuit including a pulse-width modulator. The pulse duration from the modulator is determined by the level of the output voltage from the converter. Prior to the converter reaching normal operating conditions, the power to the pulse-width modulator is generally supplied by a separate power supply operating from the alternating current source and including a step down transformer, rectifier, and filter. It will be appreciated that this starting power supply substantially increases the cost and weight of the converter. The illustrated embodiment utilizes a unique self-starting feature which allows starting of the converter without the necessity of providing a separate power supply for the start circuit.
The use of solid state devices for converter switching elements places a heavy burden on such devices which are sensitive to transient overvoltage and peak current conditions which may quickly destroy the devices. Prior converter systems have generally utilized transistors having parameters considerably in excess of those necessary to sustain normal operation so that transient conditions will be safely handled. Such an approach is costly and often increases the physical size of the cnverter due to the incorporation of overrated components. The illustrated embodiment includes means for protecting the switching device in the event of an overload condition thereby allowing its capacity to be determined by normal operating conditions rather than transient situations.
As previously mentioned, the direct current input to the converter is often provided by rectifying and filtering a commercial power source. In such arrangements, a common ground often exists between the output of the converter and the commercial power source creating an undesirable personnel hazard and potential noise source. The illustrated embodiment provides direct current isolation between the input and output terminals of the converter thus allowing the use of a commercial power source without the aforementioned disadvantages.